Field
The present disclosure relates to an acoustic lens using a Fresnel zone plate, a design method and a manufacturing method of the acoustic lens, a focusing ultrasonic transducer to which the acoustic lens is applied, and a manufacturing method of the focusing ultrasonic transducer.
Description of the Related Art
An ultrasonic transducer is a device which converts an electric signal into an ultrasonic signal or converts an ultrasonic signal into an electric signal.
The ultrasonic wave is a sound wave having a frequency higher than an audible frequency range of human hearing, that is, higher than 20 kHz. Humans cannot hear the ultrasonic wave using a sense of hearing. The ultrasonic wave is widely utilized in various fields and provides convenience in lots of areas of our life.
For example, the ultrasonic wave is used for a medical imaging diagnostic instrument. When the ultrasonic wave is used, a photograph or an image of a bodily tissue or an organ may be obtained by a non-invasive method.
Further, the ultrasonic transducer may be used to detect an external object. That is, when an ultrasonic signal is output using the ultrasonic transducer and then the output ultrasonic signal returns by being reflected from the external object, the reflected ultrasonic signal is received to measure a time taken while the ultrasonic signal returns. Presence of the external object and a distance to the external object may be calculated using the measured time.
There are three types of ultrasonic transducers that are used currently, for example, an ultrasonic transducer using a magnetic field, an ultrasonic transducer using an electric field, and an ultrasonic transducer using a piezoelectric material.
Among the three types of ultrasonic transducers, the ultrasonic transducer using a piezoelectric material is widely used because it is advantageous to reduce size and has good durability in a high frequency band (an ultrasonic band).
A piezoelectric effect is a phenomenon in which when a mechanical vibration is applied, a potential difference is generated in a crystal. Conversely, the piezoelectric effect also includes a phenomenon in which when an electric field is applied to the crystal, a mechanical vibration is generated.
Therefore, an ultrasonic transducer using a piezoelectric element generates an ultrasonic wave by a vibration generated in the piezoelectric element by applying an electric field to the piezoelectric element.
Among materials which form the piezoelectric element, Rochelle salt and quartz are single crystals, and barium titanate (BaTiO3), lead titanate (PbTiO3), and lead zirconate(PbZrO3) are multi-crystals.
When the above-mentioned piezoelectric characteristic is used, a converter for generating an ultrasonic wave, a converter for reception, or a converter for transmitting and receiving a signal may be manufactured.
In the meantime, when the ultrasonic transducer is used to detect an external object or for a visual auxiliary device for a visually impaired person, a directivity of an ultrasonic signal which is generally output is not an important factor. However, a high directivity is required to receive the ultrasonic signal.
Further, a focusing ultrasonic transducer is configured to include an acoustic lens to focus an ultrasonic wave which is excited by an ultrasonic shaker near a focal point.
FIG. 1 illustrates a cross-sectional view of an ultrasonic transducer 1 to which a spherical acoustic lens 2 of the related art is applied. As illustrated in FIG. 1, an ultrasonic shaker 10 excites an ultrasonic wave so that the ultrasonic wave enters the acoustic lens 2. The entering ultrasonic wave is focused near a focal point by the acoustic lens 2.
It is understood that an ultrasonic wave emitting surface of the acoustic lens 2 of the related art is configured by a concave surface having a predetermined radius curvature which is recessed toward an entering surface. However, in the case of the spherical acoustic lens 2 of the related art, an impedance of a material for the acoustic lens needs to be lower than an impedance of a material of the ultrasonic shaker 10 and higher than an impedance of a transmitting material. Therefore, a selectable material is limited thereto.
Further, a large thickness of the spherical acoustic lens 2 of the related art is inevitable due to the curvature radius, so that it is difficult to reduce weight and size.